1. Field of the Invention
The present invention relates to a resonator device used for, for example, a band-pass filter of a portable information terminal, a filter including the resonator device, and a duplexer. More particularly, the present invention relates to a resonator device having a plurality of resonators connected in series, a filter including the resonator device, and a duplexer.
2. Description of the Related Art
Recently, with the development of mobile communication systems, the prevalence of portable information devices such as mobile phone devices has rapidly increased. Some portable information devices utilize a plurality of high-frequency bands. For example, mobile phone devices use a transmission (Tx) band (1850 to 1910 MHz) and a reception (Rx) band (1930 to 1990 MHz). A portable information device using a plurality of high-frequency bands, such as a mobile phone device, includes a device to prevent data interference, such as a high-frequency filter including a resonator device or an antenna duplexer including a high-frequency filter.
For example, Japanese Unexamined Patent Application Publication No. 2007-36856 discloses a resonator device used for a high-frequency filter. FIG. 9 is a diagram of a resonator device 100 disclosed in Japanese Unexamined Patent Application Publication No. 2007-36856. In the resonator device 100, an inductor 102 and a capacitor 103 are connected in parallel with a resonator 101. Thus, as shown in FIG. 10, a first anti-resonance point in a frequency range higher than a resonance frequency and a second anti-resonance point in a frequency range lower than the resonance frequency are defined. Further, the frequencies of the first and second anti-resonance points can be shifted to higher or lower frequencies by adjusting the inductance of the inductor 102 and the capacitance of the capacitor 103. Therefore, a filter including the resonator device 100 has improved design flexibility.
However, in a filter including the resonator device 100 disclosed in Japanese Unexamined Patent Application Publication No. 2007-36856, it is difficult to sufficiently increase the attenuation outside the passband. It is also difficult to reduce the size of the resonator device 100.